Title says it all, and
this post over on the Flash Platform blog provides lots of details.

Some of the features and performance enhancements included in the Flash Player 11 and AIR 3 betas:

Captive Runtime for AIR*: This popular feature from AIR for iOS will now be available on AIR for desktop. Simplify the app installation process, reduce testing and certification cost by encapsulating the AIR runtime in your Windows, Mac and and Android apps.

Native 64-bit support for Flash Player: Take advantage of new 64-bit operating systems and 64-bit Web browsers on Linux, Mac OS, and Windows.

Stage 3D graphics rendering: Stage 3D (“Molehill) is a new method of 2D and 3D rendering and is supported with a new Stage3D API. The Stage3D API is a set of low-level GPU-accelerated APIs enabling advanced 2D and 3D capabilities across multiple screens and devices (desktop, mobile and tv). These new low-level APIs will provide 2D and 3D engine developers the flexibility to leverage GPU hardware acceleration for significant performance gains. You can view examples on the Stage 3D page.

Native JSON (JavaScript Object Notation) support: Enables ActionScript programmers take advantage of fast parsing and generation of JSON-formatted data. Take existing data and integrate it with minimal or no modification into your ActionScript project.